The ability to control the buoyancy of an object is desirable in many applications. For example, in the field of unmanned underwater vehicles (UUVs), it is often desirable to adjust the buoyancy of the vehicle to stabilize it in the water column (hover) or to make the vehicle rise or sink within the column.
Accordingly, many waterborne objects are provided with a buoyancy control mechanism, or “buoyancy engine”, that allows active control of the buoyancy of the object. Active buoyancy control allows the buoyancy of an object to be adjusted as necessary for a desired maneuver or to accommodate unknown or changing environmental conditions. For example, the buoyancy of the object may be adjusted to bring a submerged object to the surface so that it can communicate via radio, then return the object to a submerged condition. As another example, the buoyancy of an object might be adjusted to accommodate variations in density of the surrounding water due to changes in temperature and/or salinity.
The present application is generally applicable to any type of waterborne object for which buoyancy control is desirable. Examples of waterborne objects that employ or may employ buoyancy control include: floats, buoys, weaponry (torpedoes), and manned and unmanned powered submarines. The present invention is, however, of particular significance when applied to the class of UUV's referred to as “gliders”. A glider is propelled through the water completely by changes to the buoyancy of the vehicle. The present invention will be described in detail below in the context of a glider.
For UUVs that are powered by batteries or other fixed energy storage mechanisms, one design goal is to optimize the energy efficiency of all onboard systems. The buoyancy control engine can be a major consumer of stored energy, so an effective buoyancy control engine should be energy efficient. The buoyancy engine should also be reliable, low weight, and easily maintainable.
Conventional gliders have a buoyancy engine that effectively changes the volume of the glider. One class of conventional gliders (e.g., the “Seaglider” produced by the University of Washington and the “Spray” produced by Bluefin Robotics) uses hydraulic pumps to transfer hydraulic fluid from an internal bladder to an external bladder. Yet another class of gliders (e.g., the “Slocum Thermal” produced by Webb Research) harvests the thermal energy of the ocean to move a transfer fluid between an internal bladder and an external bladder. The buoyancy engines employed by these gliders will be referred to as “internal bladder/external bladder” buoyancy control engines.
Another class of gliders (e.g., the “Slocum Electric” produced by Webb Research) uses a motor to drive a ball screw. The ball screw in turn drives a piston inside a rolling diaphragm. The diaphragm/piston combination displaces water when extended and ingests water when retracted. This type of buoyancy engine will be referred to as “ball screw/piston” type buoyancy control engines.
A related class of UUVs includes floats or buoys (e.g., The “ALACE” (Autonomous Lagrangian Circulation Explorer) floats). In the case of floats or buoys, the purpose of the buoyancy control system is typically to maintain neutral buoyancy for a period of time at a predetermined depth and then adjust the buoyancy to cause the vessel to surface and communicate data. After the communication process is completed, the buoyancy of the vessel is again adjusted to cause the float or buoy to descend and then become neutrally buoyant at the predetermined depth. Such floats or buoys also use an “internal bladder/external bladder” configuration to control buoyancy.
One problem with the “internal bladder/external bladder” class of buoyancy engine is that a large amount of fluid is required to adjust the buoyancy of the device. Because the fluid is transferred into a bladder that directly displaces the water, there is a one to one ratio between required fluid and potential displacement (i.e. one liter of fluid is required to displace one liter of water). The ratio of required fluid to potential displacement limits the net buoyancy of the vehicle. In the context of gliders, this limitation on net buoyancy limits the speed of the glider and also the ability of the glider to adjust its buoyancy in response to changes in salinity and temperature.
Another disadvantage of the “internal bladder/external bladder” buoyancy engine is that the hydraulic pumps used in these designs are typically optimized for maximum efficiency at a significantly higher pressure than the operational pressure of the device. In particular, the hydraulic pump does not operate at maximum efficiency at the maximum operational depth of the vessel, and the hydraulic pump is even less efficient at shallower depths.
For example, the “Seaglider” glider developed by the University of Washington employs the Hydro LeDuc model PB32.5 pump. This pump has a maximum total efficiency (combined mechanical and volumetric efficiency) that peaks at approximately 34 MPa (˜5000 psi), while the pressure at the Seaglider's maximum operational depth of approximately 1,000 m yields a pressure of approximately 10 MPa (˜1500 psi). The efficiency of the buoyancy engine of the “Seaglider” glider is less than 15% at 200 m operation and only 40% at 1000 m operation.
The “ball screw/piston” type of buoyancy engine similarly suffers from low efficiency. Small DC motors are typically designed to run at high speeds (e.g. 5,000-10,000 rpm). While these motors can be highly efficient (typically 80-90%) at these relatively high operational speeds, the speed of such motors needs to be significantly reduced to drive a ball screw assembly of a “ball screw/piston” type buoyancy engine. A reduction gear is thus typically used to reduce the speed of the motor; a reduction gear is usually about 70% efficient, giving a combined efficiency in the range of 56-63%. In addition, the ball screw assembly itself typically operates at only about 95% efficiency, thereby reducing the maximum potential efficiency of this system to a range of 50-60%. The “Slocum Electric” device produced by Webb Research, which uses a ball screw/piston type buoyancy engine, has a published buoyancy engine efficiency of about 50%, which is at the low end of the theoretical range of efficiencies for the “ball screw/piston” type of buoyancy engine.
It is therefore an object of the current invention to provide buoyancy control systems and methods for a submersible vessel having improved efficiency over the entire operational depth range of the vessel. An additional object of the current invention is to provide buoyancy control systems and methods that are reliable and easy to manufacture and maintain.